Compound such as chalk a litmus test reveals (6)
I believe the answer is:
alkali
'compound' is the definition.
(I know that alkali is a type of compound)
'chalk a litmus test reveals' is the wordplay.
'reveals' indicates a hidden word.
'ALKALI' is hidden within 'chalk a litmus test'.
'such as' acts as a link.
(Other definitions for alkali that I've seen before include "Acid neutraliser" , "Soluble compound that turns litmus paper blue" , "Compound like lime" , "Salt" , "In pH terms, not acid" .)